The use of unsubstituted aryl acridinium esters as chemiluminescent labels is disclosed by J. S. Woodhead et al first in GB 2,008,247 B, and again in European Patent Application No. EP 82,636. However, these compounds did not have any substituents on the ortho positions of the phenoxy ring which constitutes part of the aryl ester component. Unfortunately the Woodhead compounds were not stable in pH 7.4 buffer media, and thus, not useful for commercial assays.